Dragging of USercontrol does not Work when Control Added while runtime

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C#

WPF

Hello,
i have created a dragabke Usercontrol. It should be Possible to drag it around in a Canvas. On the User Control i have implemented the logic for Drag and drop using three Mouse Events:

bool isMouseCaptured;
double mouseVerticalPosition;
double mouseHorizontalPosition;

public void Handle_MouseDown (object sender, MouseEventArgs args) 
{
    Rectangle item = sender as Rectangle;
    mouseVerticalPosition = args.GetPosition(null).Y;
    mouseHorizontalPosition = args.GetPosition(null).X;
    isMouseCaptured = true;
    item.CaptureMouse();
}

public void Handle_MouseMove(object sender, MouseEventArgs args) 
{
    Rectangle item = sender as Rectangle;
    if (isMouseCaptured) 
    {

        // Calculate the current position of the object.
        double deltaV = args.GetPosition(null).Y - mouseVerticalPosition;
        double deltaH = args.GetPosition(null).X - mouseHorizontalPosition;
        double newTop = deltaV + (double)item.GetValue(Canvas.TopProperty);
        double newLeft = deltaH + (double)item.GetValue(Canvas.LeftProperty);

        // Set new position of object.
        item.SetValue(Canvas.TopProperty, newTop);
        item.SetValue(Canvas.LeftProperty, newLeft);

        // Update position global variables.
        mouseVerticalPosition = args.GetPosition(null).Y;
        mouseHorizontalPosition = args.GetPosition(null).X;
    }
}

public void Handle_MouseUp(object sender, MouseEventArgs args) 
{
    Rectangle item = sender as Rectangle;
    isMouseCaptured = false;
    item.ReleaseMouseCapture();
    mouseVerticalPosition = -1;
    mouseHorizontalPosition = -1;
}

If i add the User Control form VS Toolbox to my Project, everything works fine, but
if i add the control During Runtime with:

myControl ctrl = new myControl();
myCanvas.Children.Add(ctrl);

Dragging does not work. I can`t understand it.
Any suggestions what is wrong, or what to do?

greets